A pan-European digital cellular radio system which is in use in Europe (Groupe Speciale Mobile or GSM) implements a discontinuous reception (DRX), or "slotted paging mode", when operating in an idle mode. In this mode a radiotelephone, also referred to herein as a mobile station, does not continuously monitor a paging channel when in the idle mode. Instead, the radiotelephone is required to monitor the paging channel only during an assigned paging channel time slot. During all other paging channel time slots the radiotelephone can place itself into a low power mode of operation, such as by removing power from selected circuitry, thereby reducing power consumption and prolonging battery life.
A convenient method to make power consumption comparisons for the DRX mode employs the duty cycle, or the ratio of receiver on to off (sleep) time. Clearly, the lower the duty cycle the less time the radiotelephone is required to be powered on. A reduction in the on time, or conversely an increase in the off or sleep time, provides a reduction in power consumption and an increase in battery life.
In GSM, the idle mode is based on the concept of multiframes, each of which is 235 milliseconds. (ms) long. The radiotelephone is required to read one paging message every two to nine multiframes (470 ms to 2.1 seconds), as specified by the base station. In addition, each paging message consists of four frames, where a frame is 4.615 ms in duration and consists of eight timeslots, each having a duration of 0.5769 ms. A radiotelephone is only required to receive one timeslot per frame. Therefore, the radiotelephone is required to receive only one paging message, of 18.46 ms (4.times.4.615 ms) duration, every 470 ms to 2.1 seconds. Of this 18.46 ms, the receiver circuitry is required to be active for only 18.46/8=2.31 ms. Assuming for this example that the receiver circuitry is on for the minimum time, the duty cycle ranges from a maximum of 2.31 ms/470 ms=3.9% to a minimum of 18.46 ms/2.1 seconds=0.9%.
DRX has also been proposed to be implemented in the Japanese Digital Cellular (JDC) system. Although the U.S. digital cellular TDMA system (IS-54) does not, at present, implement a DRX mode, one is under consideration for the future, using the GSM and the proposed JDC systems as models.
In the proposed implementation of DRX in the JDC system, and similar to GSM, the concept of superframes in used. Each superframe is 720 ms in duration, and consists of 36 frames of 20 ms each. The radiotelephone is required to read one paging message per superframe. Each frame in JDC consists of three timeslots, and the radiotelephone is required to receive only one timeslot of the three. In that a paging message consists of only one timeslot, of 20 ms/3=6.67 ms duration, the duty cycle in this proposed system is 6.67 ms/720 ms, or 0.93%.
As originally proposed for the U.S. Code Division Multiple Access (CDMA) system, the radiotelephone must periodically receive one 200 ms slot, as determined by a SLOT.sub.-- CYCLE.sub.-- INDEX value. The index is selected by the radiotelephone, except that the base station can set the maximum index to correspond to as small as a one second cycle time. A typical, reasonable slot cycle for a radiotelephone is two seconds. Therefore, the duty cycle could be as low as 200 ms/2 seconds=10%, and as high as 200 ms/1 second=20%. Both of these duty cycle values are clearly significantly greater than the corresponding minimum and maximum values achievable with the GSM and the proposed JDC systems.
In addition, there exists a certain amount of overhead to receive a slotted page message. Because of continuous convolutional coding on the CDMA paging channel, the radiotelephone must receive at least a frame before and after the 200 ms slot, depending on the paging channel data rate. This time, in conjunction with various turn-on times in the radiotelephone receiver, results in a typical overhead of up to 100 ms. The total on-time of the radiotelephone thus becomes approximately 300 ms, resulting in a duty cycle between 20% and 30%, depending on the slot cycle length.
Furthermore, it is possible that the radiotelephone would be required to receive two paging channel slots. This can occur if the base station uses the MORE.sub.-- PAGES bit in the SLOTTED PAGE MESSAGE, thereby requiring the radiotelephone to receive up to one additional slot. Also, the CDMA specification states that the radiotelephone may stop listening to the paging channel after reading the SLOTTED PAGE MESSAGE, there is no guarantee that this message is located at the beginning of the slot. As a result, it may happen that the radiotelephone must always listen to the entire slot.
As presently specified for use, a CDMA radiotelephone includes a system time pseudonoise (PN) generator, also referred to herein as a "short code" as opposed to "long code" generator. The system time short code PN generator has a rollover period of 26.67 milliseconds, and is aligned with the frame timing (20 milliseconds) every 80 milliseconds.
Another feature of the CDMA system is the use of a Long Code for mobile unit identification. The Long Code is a PN sequence with period 2.sup.42 -1 that is used for scrambling on the Forward (base station to mobile) CDMA Channel and for spreading on the Reverse (mobile to base station) CDMA Channel. The Long Code uniquely identifies a mobile station on both the Forward and Reverse Traffic Channels. The Long Code also serves to provide limited privacy, and separates multiple Access Channels on the same CDMA channel. A Long Code Mask is a 42 bit binary number that creates the unique identity of the Long Code.
A problem is created when it is desired to periodically shutoff a long code generator, such as when powering down the radiotelephone when operating in the slotted paging (DRX) mode described above, and to then restart the long code generator in the proper state when powering back up. In that the long code generator is intended to run continuously, it is essential that the long code generator be initialized to the proper state whenever it is started after a period of non-operation.
One method has been proposed which would read the state of the long code generator just prior to powering down the mobile station. A complex matrix multiply operation is then applied to the long code to determine the correct state of the long code generator at a time in the future when the long code generator is to be reinitialized.
However, this approach is computationally expensive. As a result, it may be necessary to "wake up" the radiotelephone earlier than would be necessary if the complex matrix multiply operation is performed after the power down period. If the matrix multiply is performed before powering down, then the mobile station must remain in a powered up state for a period of time sufficient to accomplish the matrix multiply. In either case, the mobile station is powered on for a longer time. The causes the overall duty cycle and power consumption to increase, thus decreasing battery life.